Muting system



Oct. 3, 1939.

D. V. SINNINGER MUTIHG SYSI'EI Filed Aug. 9. `195'7 M ATTORNEY PatentedOct. 3, 1939 UNITED STATES PATENT OFFICE MUTIN G SYSTEM poration ofIllinois Application August 9, 1937, Serial No. 158,073

4 Claims.

This invention relates to radio receiving systems and more particularlyto means for preventing response from a radio receiver while it is beingadjusted to receive a desired signal.

5 In the past, in order to secure silent tuning with the conventionaltype of radio receiver it was necessary to retard the manual volumecontrol prior to and during the tuning operation and to then advance itto the desired point upon completion of the operation. Thus threeseparate steps were necessary in order to avoid the annoyance of hearinga mixture of undesired signals and extraneous noise when changing fromone setting of the station selector to another.

Since most modern radio broadcast receivers are provided with Visualmeans for indicating exact resonance with all signals of usefulstrength, it is generally unnecessary that the receiver produce anyaudible response during the tuning operation. It remains, therefore, toprovide automatic means for muting the receiver while the stationselector is actually in use, so that no sound will be heard until thetuning operation is Completed.

It is an object of this invention to provide simple means forautomatically muting a radio receiver during the tuning operation.

A further object of the invention is to provide automatic muting meanswhich will effect continuous muting during tuning operations of a radioreceiver persisting through short interruptions that may occur in any ofsaid tuning operations.

It is a further object of the invention to greatly 5 simplify the tuningoperation in radio receivers,

by automatically preventing all extraneous noises during the operation,so that the user of the receiver may readily and without distractionadjust the instrument closely to exact resonance with,

and consequently to the position of best possible response to, thesignal of the desired station.

It is also an object of the invention to greatly simplify thearrangements for producing satisfactory muting of the receiver duringthe tuning operation, and to thus reduce not only the cost but also thecomplication and liability to service failure thereof.

It is an additional object of the invention to 50 provide a mutingarrangement by which the operator may listen momentarily at will to thesignals from each of a number oi different stations as he tunes thereceiver, to determine which of them he desires to listen to, withoutthe annoy- 55 ance of static or other unwanted noises, and with- (Cl.Z-20) out hearing the signals of any stations except those which heelects to hear.

In accordance with a preferred form of the invention, contact of theoperators fingers with the knob which actuates the station selector of 5the receiver is utilized to actuate electrically operated means formuting the receiver. The muting is accomplished by rendering anaudiofrequency amplifying vacuum tube substantially inoperative bybiasing it beyond cut-off, so that 10 the response of the receiver isreduced practically to zero. In the arrangement according to theinvention, the audio-frequency vacuum tube is either operating normallyor is completely cut off, and thus no distortion of the signal is 15introduced by the addition of my muting system.

The invention will be better understood by reference to the accompanyingdrawing, in which:

Fig. 1 is a schematic circuit diagram of a preferred form of theinvention; and 20 Figs. 2, 3, 4 and 5 are elevations, partly in section,of various forms of selector or tuning knobs which may be employed inconnection with the invention.

Referring to Fig. l of the drawing, block I 25 represents thehigh-frequency amplifying and selecting means, and the detector, of aradio receiver. Grid 2 of 'audio-frequency amplifying vacuum tube 3 isconnected to the output of unit I by means of blocking capacitor 4.Cathode 5 30 of vacuum tube 3 is grounded through resistors 6 and 'I inseries, which are shunted by capacitors 8 and 9, respectively. The lowerterminal of grid-leak Il] is connected to the junction of resistors Eand 'l through resistor II, which is 35 shunted by capacitor I2.Resistor Ill is connected between the junction of grid-leak I0 andresistor II and one terminal of the device I5, the other terminal ofwhich is grounded. Device I5 may be a modified tuning knob presently to40 be described. Device I 5 is indicated as being mechanically linked tounit I by dotted line I6. Which, in the usual case, Will be the drivingshaft of a variable condenser gang or other tuning unit. A switch I3 maybe provided between re- 45 sistors II and I4 to render the muting deviceinoperative if desired. The plate circuit of vacuum tube 3 includes loadresistor Il, and bleeder resistor I8 is connected between the junctionof resistors B and 'I and a source of positive plate 50 voltage, notshown. Resistors l and I8 are so chosen that the potential drop acrossresistor 'I due to current from the positive plate-voltage source issuicient at all times to bias vacuum tube 3 well beyond cut-off. Whenthis potential 55 is applied to grid 2, the cathode current of vacuumtube 3 becomes practically zero and therefore the potential drop acrossresistor 6 is negligible, and under these conditions, vacuum tube 3ceases to amplify.

In operation, grid 2 of vacuum tube 3 is normally provided with thecorrect bias voltage due to the potential drop across resistor 6 in itscathode circuit. The audio-frequency path from unit I to ground includescapacitor 4, grid-leak Il), capacitor I2, and capacitor 9. The drop inaudiofrequency potential across grid-leak I is applied to grid 2 ofvacuum tube 3, and appears in amplied form across load resistor I'I inthe plate circuit of vacuum tube 3.

If switch I3 is absent, however, or if it is present and closed, and thecircuit is completed through device I in a manner to be described below,grid 2 of vacuum tube 3 is effectively at ground potential from thedirect-current standpoint. Therefore the effective grid-bias voltagebecomes equal to the potential drop across resistor I, and vacuum tube 3ceases to function as an amplifier because it is biased beyond cut-off.Device I5 is preferably so arranged that the fingers of the operatorwhen he touches the knob which actuates the station selector provide arelatively low-resistance path from resistor I4 to ground. In this way,the receiver is muted during the tuning operation.

It will be understood that switch I3 may be operable from the frontpanel of the receiver, so that the muting system may be renderedinoperative if desired. It is also within the scope of the invention,when applied to a receiver adapted to cover more than one band offrequencies, to prol vide means for actuating switch I3 simultaneouslywith the band-selecting device of the receiver, in such a manner thatthe muting system is automatically rendered inoperative in one or moreof the frequency bands covered by the receiver.

Although the muting system of the invention operates substantiallyinstantaneously upon completion of the circuit by device I5, it isdesirable that the muted condition be maintained for a short period oftime after the tuning knob is released, so that it is unaffected bymomentary interruptions of contact by the operators hand, as for exampleto secure a new grip on the knob for further rotation. In accordancewith the invention, this desirable delay may be realized by so choosingthe values of resistor II and capacitor I2 as to provide a suitable timedelay in the restoration of normal bias voltage to grid 2.

Since in some forms of my invention, the conductive connection which isestablished by device I5 includes a portion of the operators fingers,resistor I I is preferably of high resistance relatively to theresistance of the conductive path established by device I5 under theseconditions, and thus the provision of a suitable time delay requires theuse of only a small value for capacitor I2. Resistor I4 functions merelyto isolate device I5 from potentials greater than ground, in order toavoid the possibility of the operator receiving a slight shock duringthe tuning operation.

Referring now to Figs. 2, 3, l and 5, each of these figures shows apossible form of device I5 of Fig. l. Figs, 2, 3 and 4 each showarrangements which may be utilized as the tuning knob which actuates theshaft that drives the variable condensers or other ganged tuningdevices.

Fig, 5 shows an arrangement which may be used independently of thetuning knob.

In Fig. 2, I9 is a knob of wood or other insulating material. Aroundthat portion of the knob I9 with which the ngers of the operator comeinto direct contact are two coaxial conductive rings 2l] and 2I, ofWire, fiat strip or other suitable material. Rings 29 and 2I may bemolded into knob I9, or may be applied in suitable grooves after theknob is made. Ring 29 is connected to shaft 22 upon which knob I9 ismounted. Ring 2l is connected to a slip-ring 23 mounted on the rearsurface of knob I9, and a connection to ring 2l is secured through brush24 which bears on slip-ring 23. Shaft 22 is journaled in bushing 25 inthe front panel of the receiver, which provides a connection to ring 2U.

In Fig. 3, shaft 22 carries at its outer end punched or deep-drawn metalknob 25 which has a knurled periphery. Bushing 2'I extends in front ofthe receiver panel and carries insulating sleeve 23 upon which ismounted stationary conductive knob-shaped member 29 which has a smoothperiphery, and is of substantially the same diameter .as knob 26. Member29 and knob 26 are so positioned that the fingers of the operatorreadily bridge the small gap separating them` and provide a relativelylow-resistance connection between them. When the operator attempts torotate knob 2G and member 29, knob V2t is rotated by virtue of itsknurled periphery but member 29 remains stationary, since it is not freeto rotate, and its smooth periphery exerts practically no restraintagainst the operators turning effort. A connection 30 is providedthrough a suitable hole in the receiver panel to member 29.

In Fig. 4, shaft 22 carries at its outer end punched or deep-drawnconductive knob 3l, which has a knurled periphery. Secured to shaft 22,for example by a forced t, there is an insulating sleeve 32 upon whichis mounted, also preferably by a forced t, conductive member 33 whichhas a knurled knob-shaped flare 34 of substantially the same diameter.as knob 3l at its outer end and a small shoulder 35 as shown. Member 33is positioned so as to leave a small air-gap to knob 3l. Parts 22, 3|,32 and 33 together constitute av rotatable knob and shaft, the knobhaving two insulated peripheral surfaces. Member 33 is journaled inbushing 3G in the panel of the receiver, which provides a connectionthereto. Post 31 mounted on the rear of the receiver panel carries brush38 which bears on shaft 22 and provides a connection to knob SI.

In Fig. 5, Contact fingers 39 .and 4E! are arranged to be closed bypressure on knob 4I, but are normally held out of contact by spring 42.In this arrangement, when the operator desires to tune the receiver, hedepresses knob 4I, thus closing contacts 39, 49, and when the tuningoperation is completed and the receiver is accurately tuned to thedesired station, he releases knob 4I. It will be understood that bysuitable mechanical arrangements knob 4I may be the tuning knob of thereceiver, so that upon rotation it actuates the tuning device in thereceiver.

The various structures shown in Figs. 2, 3, 4 and 5 are given merely byway of illustration of suitable devices for use in carrying out myinvention, and other structures for securing an electrical circuitthrough the lingers of the operator or as the result of his actuation ofthe controls of the receiver will readily occur to thdcse skilled in themechanical arts and are to be taken as the full equivalents of thestructures shown. Additionally, electrically operated contacts, actuatedfor example by a relay device, may also be substituted.

The operation of a receiver equipped with muting arrangements accordingto the invention is highly pleasing and successful. When the operatordecides to tune the receiver to a new station, he places his fingersupon the tuning knob or other suitably arranged control, and thereceiver is immediately muted, that is, the signal from the station towhich the receiver was previously tuned becomes inaudible, and the loudspeaker is silent. The operator then turns the knob to tune to the newstatio-n, watching the visual tuning indicator, or, if no indicator isprovided, then the markings on the receiver dial, until he is sure thatthe receiver is now properly tuned to the new station. He then takes hisfingers off the knob, and, after a very short delay due to the timeconstant of the circuit, as eX- plained above, the signal of the newstation bccoines audible.

If, during the tuning operation, the operator removes his fingers fromthe knob toV get a new grip upon it to make further rotation possible,the time constant of the circuit may be and preferably is arranged to besuch that during the short interval in which the operators fingers arenot in contact with the knob, the receiver remains muted and no sound isheard from the loud speaker.

An additional advantage of the invention is that with a receiver soequipped it is readily possible for the operator to listen momentarilyto the signal from each of a number of stations as he tunes from one endof the frequency range to the other, by merely removing his ngers fromthe tuning knob, to determine whether the content of the signal from anyone of them interests him particularly at the moment, or to decide whichof the available stations he will listen to, all without the distractionof unpleasant noises and sounds from unwanted stations during the tuningoperation.

Having thus described my invention, what I claim is:

l. A radio receiver including tunable circuits; a station selector fortuning said circuits; an audio-frequency amplifying vacuum tube having agrid, a cathode and a plate; two adjacent mutually insulated conductivesurfaces upon the ringer-grip portion of said station selector; twogrid-circuit resistors in series between said grid and one of saidsurfaces; two cathode resistors in series between said cathode and theother of said surfaces; by-pass capacitors across each of said cathoderesistors; two plate-circuit resistors in series between said plate andthe junction of said cathode resistors; a connection from the junctionof said plate-circuit resistors to a source of plate potential; and apath through the fingers of the operator between the junction of saidgrid-circuit resistors and the junction of said cathode resistors havinghigh direct-current resistance and low impedance.

2. A radio receiver including tunable circuits; a station selector fortuning said circuits; an audio-frequency amplifying vacuum tube having agrid, a cathode and a plate; two adjacent mutually insulated conductivesurfaces upon the finger-grip portion of said station selector adaptedto be electrically connected through the 1ingers of the operator; twogrid-circuit resistors in series between said grid and one of saidsurfaces; two cathode resistors in series between said cathode and theother of said surfaces; bypass capacitors across each of said cathoderesistors; two plate-circuit resistors in series between said plate andthe junction of said cathode resistors; a connection from the junctionof said plate-circuit resistors to a source of plate potential; and aresistor and a capacitor connected in parallel between the junction ofsaid grid-circuit resistors and the junction of said cathode resistors.

3. A radio receiver including tunable circuits; a station selector fortuning said circuits; an audio-frequency amplifying vacuum tube having agrid, a cathode and a plate; two adjacent mutually insulated conductivesurfaces upon the finger-grip. portion of said station selector adaptedto be electrically connected through the nngers of the operator; twogrid-circuit resistors in series between said grid and one of saidsurfaces; two cathode resistors in series between said cathode and theother of said surfaces; bypass capacitors across each of said cathoderesistors; two plate-circuit resistors in series be tween said plate andthe junction of said cathode resistors; a connection from the junctionof said plate-circuit resistors to a source of plate potential; and aresistor and a capacitor connected in parallel between the junction ofsaid grid-circuit resistors and the junction of said cathode resistors;said resistors being of such value as to provide efcient operating biasupon said grid but such as to materially increase the negative bias uponsaid grid upon manual grasping of said station selector.

4. A radio receiver including tunable circuits; a station selector fortuning said circuits; an audio-frequency amplifying vacuum tube having agrid, a cathode and a plate; two adjacent mutually insulated conductivesurfaces upon the finger-grip portion of said station selector adaptedto be electrically connected through the fingers of the operator; twogrid-circuit resistors in series between said grid and one of saidsurfaces; two cathode resistors in series between said cathode and theother of said surfaces; bypass capacitors across each of said cathoderesistors; two plate-circuit resistors in series between said plate andthe junction of said cathode resistors; a connection from the junctionof said plate-circuit resistors to a source of plate potential; saidresistors being of such value as to provide efficient operating biasupon said grid but such as to materially increase the negative bias l,

upon said grid upon manual operation of said station selector; and aresistor and a capacitor connected in parallel between the junction ofsaid grid-circuit resistors and the junction of said cathode resistorsand of such values as to maintain said bias at said increased valueduring momentary interruption of said manual operation.

DWIGHT V. SINNINGER.

